This invention relates to an improved process for selectively removing hydrogen sulfide from gaseous streams. More particularly, this invention relates to an improved process for selectively removing hydrogen sulfide from gaseous streams containing less than 500 ppm free hydrogen, by contacting said gaseous streams with a nickel-promoted absorbing composition.
The removal of sulfur from fluid streams can be desirable or necessary for a variety of reasons. If the fluid stream is to be released as a waste stream, removal of sulfur from the fluid stream can be necessary to meet the sulfur emmission requirements set by various air pollution control authorities. Such requirements are generally in the range of about 10 ppm to 500 ppm of sulfur in the fluid stream. If the fluid stream is to be burned as a fuel, removal of sulfur from the fluid stream can be necessary to prevent environmental pollution. If the fluid stream is to be processed, removal of the sulfur is often necessary to prevent the poisoning of sulfur sensitive catalysts or to satisfy other process requirements.
A variety of methods employing regenerable, solid contact materials are known for removing sulfur from a fluid stream when the sulfur is present as hydrogen sulfide. For example, U.S. Pat. No. 4,371,728 discloses a composition comprising zinc, titanium and at least one metal promoter which is an effective absorbing composition for hydrogen sulfide and which possesses the property of being regenerable to the original absorbing composition state in the presence of oxygen when fully sulfided. Also, U.S. Pat. No. 4,725,415 discloses a composition comprising zinc titanate, alumina, at least one metal promoter selected from tungsten and molybdenum, and at least one metal promoter selected from Group VIII of the Periodic Table which is an effective absorbing composition for hydrogen sulfide and which possesses the property of being regenerable to the original absorbing composition state in the presence of oxygen when fully sulfided.
Although the compositions disclosed in these U.S. patents are effective absorbing compositions for hydrogen sulfide, it has been found that when the hydrogen sulfide containing fluid stream has a free hydrogen content below about 500 ppm, many of the promoting metals employed in these compositions effectively oxidize significant amounts of the hydrogen sulfide to sulfur dioxide. The resulting sulfur dioxide is not absorbed by these patented compositions and, thus, passes unabsorbed through the contact material. In view of the fact that environmental concerns are focused on the total amount of sulfur contained in an effluent stream, and not just the amount of hydrogen sulfide, passing sulfur dioxide through the contact material and out to the environment is not acceptable under current environmental standards.